1. Field of the Invention
The present invention relates to an improved interrupting device or interrupter and an improved current-limiting arrangement that enables the transfer of current from a main-current-path section into a current-limiting section comprising a plurality of fusible elements. The present invention is an improvement over the arrangements disclosed and claimed in the following, commonly assigned U.S. Pat. Nos.: 4,359,708; 4,481,495; and 4,467,307.
2. Description of the Related Art
The aforementioned U.S. Pat. Nos. 4,359,708 and 4,481,495 are directed to high-voltage interrupting devices and current-limiting fuses. As discussed in those patents, the fusible elements disclosed therein are suitable for use in parallel with the main current path of an interrupting device; one or more fusible elements comprising the current-limiting section of the interrupting device. An interrupting module utilizing one or more of the fusible elements of the aforementioned patents and a main-current-path section including a switch are disclosed in the aforementioned U.S. Pat. No. 4,467,307. The interrupting device as shown in U.S. Pat. No. 4,467,307 may include a common housing for the main-current-path section and the one or more fusible elements; the one or more fusible elements being intimately surrounded by compacted fulgurite-forming medium such as silica or sand. The main-current-path section has a high continuous current rating and is designed to carry much higher continuous currents than the current-limiting section. The impedance of the main current path is much lower than the impedance of the current-limiting section so that under normal conditions, with the main current path closed, the main current path carries substantially all the current with only a negligible portion of the current flowing through the one or more fusible elements of the current-limiting section. When an overcurrent condition exists, the switch in the main-current-path section is operated to thereby create one or more gaps in the main current path. Upon the creation of the one or more gaps, the current then commutates or transfers to the current-limiting section whereupon the one or more fusible elements melt and interrupt the current according to well-known principles.
The fusible element in U.S. Pat. No. 4,359,708 includes spaced groups of holes with the separation between adjacent holes within each group being substantially less that the separation between adjacent groups of holes. This arrangement provides for suitable control of the rate of rise of the arc or back voltage after the fusible element has been completely burnt back in the area between the individual holes within each group of holes; burn-back or melting of the fusible element thereafter continuing between the groups.
The fusible element of U.S. Pat. No. 4,481,495 also includes groups of holes or notches with the width of the fusible element being greater between the groups of holes or notches. Accordingly, the fusible element of the patent makes use of the merged arcs within the groups of holes or notches as in the U.S. Pat. No. 4,359,708. Further, after the arcs have merged within the group, the rate of burn back of the ribbon is further slowed due to the decrease in current density in the element at the wider regions. Accordingly, the back voltage across the device is prevented from exceeding a selected value.
The interrupting device, for example, as disclosed in U.S. Pat. No. 4,467,307, is required to transfer or commutate high currents from the main current path to the one or more fusible elements of the current-limiting section. Specifically, the maximum instantaneous current that can be transferred into the one or more fusible elements can be a limiting factor regarding the maximum interrupting capability of the interrupting device and the capability to interrupt high-frequency currents. For suitable operation of the interrupting device at higher operating voltages, the length of the fusible elements and the components of the main current path are increased. Accordingly, the rapid transfer of current to the one or more fusible elements is adversely affected due to the increase of the impedance of the one or more fusible elements and the reduced velocity of the movable portion of the switch in the main-current-path section. Of course, if melting of one or more of the fusible elements takes place before the transfer of current from the main current path to the current-limiting section is complete, successful interruption may not be accomplished.
Arrangements to improve the transfer of current from the main current path to the one or more fusible elements are disclosed and claimed in copending, commonly assigned U.S. application Ser. Nos. 791,178 and 810,792 filed in the name of Roy T. Swanson. The application Ser. No. 810,792 includes first and second contacts arranged in telescoping fashion such that, upon rapid movement of the first contact relative to the second contact, there is a delay in the creation of the gap to allow the movable contact to accelerate to desirably high velocities before significant arc voltages are generated by the creation of the gap. In the application Ser. No. 791,178, energy absorbing means, such as a member fabricated from arc-extinguishing material, is provided between an insulative piston and a movable contact; the piston being arranged to be moved at high speeds to drive the movable contact through the energy-absorbing member thereby preventing undesirable dynamic interactions and rebounding between the piston and the movable contact.
Either as an alternative to or as an extension of the arrangements in these co-pending applications, to alleviate the difficulties encountered with increasing voltage ratings related to the transfer of current from the main current path to the fusible elements, the melting time of the fusible elements can be increased such that higher heating effects of the current (I.sup.2 t) are required before the fusible elements melt. The increase in the I.sup.2 characteristic of the fusible elements can be achieved by using thicker or wider fusible elements or by increasing the number of fusible elements in the current-limiting section of the interrupting device. Increasing the melting I.sup.2 t of the fusible elements provides additional time for the transfer of current from the main current path to the current-limiting section before melting takes place.
While the increase in melting I.sup.2 t of the fusible elements allows additional time for the transfer of high currents from the main current path to the current-limiting section before the melting of any of the fusible elements, the minimum current that will melt the one or more fusible elements to accomplish interruption is also increased. Additionally, at low overcurrents, the time that is required to melt the fusible elements will also be similarily increased. Accordingly, increasing the I.sup.2 t to melt the fusible elements by increasing the number of fusible elements, or by using thicker or wider fusible elements, increases the minimum current that can be cleared or interrupted by the interrupting device--absent specific facilities or instrumentalities to counter the increase in the minimum clearing current.
Fusible elements including various patterns of holes or notches and/or having various cross-sectional geometries or structure are also shown in the following: U.S. Pat. Nos. 2,833,891; 2,866,040; 3,863,187; 3,909,766; 4,123,738; 4,146,863; 4,150,354; 4,219,794; 4,204,184; 4,227,167; and 4,227,168; German publication No. 1,193,154; Canadian Pat. No. 1,001,698; and Canadian Pat. No. 1,010,483.
The aforementioned U.S. Pat. No. 4,123,738 in the Summary of the Invention thereof recites a current-limiting fuse including a main fusible element with an "M" spot (body of low melting temperature alloy) adjacent thereto, and an auxiliary fusible element across the main fusible element. The current-limiting fuse is stated to include elements for significantly reducing the arcing time required to clear low-magnitude fault currents without appreciably affecting the minimum melting I.sup.2 t value, the time-current curve, or the I.sup.2 t let-through of the fuse. As shown in FIG. 3 of the U.S. Pat. No. 4,123,738, the main fusible element 38 includes a reduced section 40 at the "M" spot and reduced sections 42 and 44 at the portions at which the ends of the auxiliary fusible element 30 are connected through respective arc-gap electrodes 34,36. The sections 46, 48, 50 and 52 of the main fusible element 38 between the reduced sections 40, 42 and 44 include a plurality of uniformly-spaced circular perforations 26 to define fusion points of minimum cross-sectional areas along the fusible element 38. For small but prolonged overload currents, the fusible element 38 melts first at the "M" spot 28 (FIG. 4b of U.S. Pat. No. 4,123,738). Due to the reduced portion 40, the fusible element 38 burns back at a much faster rate than the conventional fuse element 14 of FIG. 1 of the U.S. Pat. No. 4,123,738 which is depicted as being of uniform width and as having uniformly-spaced circular perforations 26. As the arc voltage across the auxiliary element exceeds a given value, the gaps 34,36 spark over and current is diverted to the auxiliary fuse element 30 (FIG. 4c of U.S. Pat. No. 4,123,738). The arc across the "M"-spot portion of the fusible element 38 extinguishes and the fulgurite about the portion 40 cools. The intensive heat of the arcs at arc-gap electrodes 34,36 quickly burns open the reduced sections 42,44 (FIG. 4d of U.S. Pat. No. 4,123,738); the rate of burn back at the reduced portions 42 and 44 being higher than the rate of the conventional fusible element 14. When the auxiliary element 30 vaporizes (FIG. 4e of U.S. Pat. No. 4,123,738), the circuit is interrupted. For high magnitude fault currents, the reduced portions 40, 42 and 44 and the fusion points of the sections 46, 48, 50 and 52 are stated to vaporize almost instantaneously. While the arrangement of the U.S. Pat. No. 4,123,738) is alleged to provide desirable results, the manufacture thereof for consistent operation would appear to be rather complex as requiring a main fusible element of reduced crosssectional areas, an "M" spot, an auxiliary element formed by a fusible wire, and the arc-gap electrodes 34, 36. Although it is somewhat unclear, the U.S. Pat. No. 4,123,738 appears to utilize two main fusible ribbons 14 or 38 to provide the main fusible element 12 of the fuse 10. Similarly, two fusible wires 32 comprise the auxiliary fusible element 30.
While the aforementioned arrangements may be generally suitable for their intended use, it would be advantageous to provide an interrupting device with an improved and easily manufacturable current-limiting section so as to enable the transfer of higher currents from the main current path of the interrupting device into the current-limiting section without substantially affecting the minimum current that can be cleared or interrupted by the current-limiting section.